A typical robot arm comprises a series of rigid links, each of which is connected to the next by a respective articulation. Each articulation is designed to have appropriate characteristics of strength, range of motion, size etc. for the purpose the arm is to perform.
One particular application of robots is for performing or assisting in surgery. FIG. 1 illustrates a typical surgical robot arm. A patient 1 is lying on a bed 2. The robot arm 3 extends from a base 4 towards the patient. The arm has a series of rigid links 5, 6, 7, which are connected to each other and to the base by articulations 8, 9, 10. The articulations provide a sufficient range of motion that the arm can approach the patient in different ways so as to perform a range of surgical procedures. The links can be made to move about the articulations by motors 11 which are under the control of a surgeon. The final link 7 of the arm terminates in a wrist articulation 12 to which a surgical instrument 13 is attached. The surgical instrument is designed for insertion into the patient and, for example, could be an endoscope or could terminate in a cutting or pinching tool.
It is desirable for the tip of the surgical instrument to be articulated and hence mobile, so that it can be placed in a wide range of orientations relative to the remainder of the surgical instrument. That assists in allowing the surgical instrument to perform a wide range of surgical procedures, and in allowing a surgeon to place multiple arms close to a surgical site. It is also desirable for the surgical instrument articulation to be kinematically well-functioning, without there being any attitudes in the core of its range of motion that are difficult to reach or where there could be poor control over the motion of the end effector.
U.S. Pat. No. 4,257,243 describes a constant velocity joint for coupling a tractor drive shaft to an agricultural machine. U.S. Pat. No. 3,470,712 describes a similar arrangement for serving as a constant velocity coupling.